Antiskid brake control

ABSTRACT

A road vehicle antiskid brake control having a separating valve for isolating the hydraulic brake line to the wheel and a pressure relief cylinder and piston connected to the brake line to increase the volume of the line and relieve the brake pressure when the separating valve is closed. The separating valve and pressure relief cylinder are interconnected hydraulically by a pair of two-position solenoid operated valves. The movement of the pressure relief piston is independent of the separating valve and is determined by the application of a controlled pressure differential across the piston.

United States Patent 1 3,682,513 O iir [451 Aug. 8, 1972 [54] ANTISKIDBRAKE CONTROL 3,414,336 12/1968 Atkin et al ..303/2l F 72 Inventor;Heinrich obenhi-u, oifenbacb 3,586,388 6/1971 Stelzer ..303/2l FRumpenheim, Germany 3,545,817 12/1970 Yarber ..303/2l BE UX [73]Assignee: I'I'l Industries, Inc, New York, Pn-mary Examine, MihonBuchler Assistant Examiner-Stephen G. Kunin [22] Filed: July 31, 1970Attorney-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. Hemminger,Charles L. Johnson, Jr., Philip [21] Appl' 59974 M. Bolton, IsidoreTogut, Edward Goldberg and Menotti J. Lombardi, Jr. [30] ForeignApplication Priority Data Aug. 7, 1969 Germany ..P 19 40 248.3 [57]ABSTRACT A road vehicle antiskid brake control having a [52] US. Cl...'..303/21 F, 303/21 AF, 303/61 separating valve for isolating thehydraulic brake line [51] Int. Cl. ..B60t 8/02 t th wheel and a pressurerelief cylinder and piston Field of Search 303/6 6 connected to thebrake line to increase the volume of 61-63, 68-69 the line and relievethe brake pressure when the separating valve is closed. The separatingvalve and [56] References C'ted pressure relief cylinder areinterconnected hydrauli- UNITED STATES PATENTS cally by a pair oftwo-position solenoid operated valves. The movement of the pressurerelief piston IS I! wlcklm 5 independent of the separating valve and isdetermined rayer b the a lication of a controlled ressure differential3,556,609 1/1971 MacDufi ..l88/l8l A x i i p 3,574,416 4/1971 Skoyles..303/21 F 3,498,683 311970 Leiber ..188/181 A x 8 Claim, 3 DrawingFigures 4 MASTSSE WIEEL SPEED SEA 50E SKID DE 73 AltZ/NA/[A me ANTISKEDBRAKE CONTROL BACKGROUND OF THE INVENTION This invention relates toantiskid brake control systems for vehicles and particularly to systemsof this type in which the hydraulic line to the wheel brake cylinder isisolated or cut off from the pressure source and the volume of the lineincreased to relieve the brake pressure in response to a signalindicating an incipient skidding of the wheel.

In a known antiskid system of this type the movement of the piston whichaflects the volume increase in the hydraulic line is mechanicallycoupled to the movement of the separating valve which isolates the brakeline. The piston is moved by a controlled constant pressure differentialwhich acts across the diaphragm attached to the piston and in oppositionto the high preload of a spring which acts between the housing and thediaphragm and piston to return the piston to its rest position.

The high force necessary to assemble and control the high prestressedsprings is a definite disadvantage to this system since the spring forcemust exceed the brake pressure acting on the other side of the piston.In addition, the spring must be dimensioned so that the spring movesthrough the linear portion of the spring characteristic only to insurethat the volume increase in the brake circuit will be analogous to thecontrol signal. Since the pressure relief piston is mechanically coupledto the separating valve, the hydraulic fluid will be sucked from thebrake circuit whenever the separating valve is actuated. This will causean underpressure or vacuum in the brake line when the piston is actuatedwith little or no brake pressure being applied, such as when the controlis being tested without the application of the brake. The vacuum orunderpressure whichis caused may result in foaming or vaporization ofthe hydraulic fluid and a complete loss of brake pressure. Anotherdisadvantage is that the high springpressure will cause a pressure peakin the brake circuit when the piston is returned to its initial positionafter a control cycle. In addition to this, operation of the control isnoisy and the high forces involved cause premature wearing of the parts.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide an antiskid brake control system in which the separating valveand the pressure relief piston are mechanically separated and may beactuated independently and successively.

It is a further object of this invention to eliminate the necessity fora high preload spring used in prior systems.

This invention achieves these objects by providing an antiskid brakecontrol system having a hydraulically interconnected separating valveand a pressure relief piston to effect an increase in the volume of thebrake line connected to the wheel. The pressure relief piston movesindependently of the separating valve and is acsource or reservoir via apair of two-way valves. The other end of the pressure relief cylindercommunicates with the brake line which is isolated by the action of theseparating valve.

The two-way valves are similar solenoid valves having double-acting conesurfaces which connect a central annular port to one of two axiallyspaced ports. The edges of the central annular port provide the seat forthe symmetrical conical surfaces.

The actuating chamber for the separating valve is connected to one portin each valve. The other port in each valve is separately connected tothe actuating chamber of the pressure relief cylinder by means ofhydraulic lines. The central annular port of one valve is connected tothe hydraulic pressure source which may be an accumulator while theother is connected to the system reservoir.

In the rest or neutral position of the system the spring loaded solenoidvalves connect the pressure source to the actuating chamber of thepressure relief cylinder and the reservoir to the actuating chamber ofthe separating valve. The separating valve is spring loaded to theopened position in the neutral condition while the pressure acting onthe surface of the pressure relief piston prevents any volume increasein the brake line.

A check valve is fitted in the conduit between one of the two-way valvesand the actuating chamber of the pressure relief piston to prevent fluidfrom flowing back toward the pressure source.

A locking device is provided for preventing actuation of the pressurerelief piston if the pressure supplying is lost. A spring loaded pushrod designed to counteract the actuating force of the solenoid valve isheld away from the two-way valve during normal conditions by means of apressure chamber and piston surface attached to the push road, thepressure chamber being connected to the presure source. When thepressure source fails, the push road, which passes through a connectingbore in the cylinder chamber in one of the twoway valves, will move toinsure that the valve is in a position in which the fluid cannot drainout of the actuating chamber of the pressure relief cylinder when thepressure supply is lost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section of anantiskid control system embodying the present invention with the systembeing in the relaxed or deactivated condition;

FIG. 2 is similar to FIG. 1 showing the system actuated during theantiskid cycle; and

FIG. 3 is a partial sectional view showing the safety mechanism in theactuated condition.

DESCRWTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 there isshown a housing 3 in which pressure medium channels 1, from the mastercylinder, and 2, to the wheel cylinder, are connected with one anotherby a wider, stepped cylinder bore 4 in which a pressure medium operatedseparating valve 5 is arranged and combined with a check valve. Acylinder bore 6 is positioned in the pressure medium channel 2 leadingto the wheel cylinder. A pressure relief piston 7 is arranged to slidein the bore 6 and is sealed against the cylinder wall. In rest positionthe pressure relief piston 7 lies with its flange 9 on a shoulder 10 inthe cylinder bore 6. The chambers of the cylinder bores 4 and 6 whichare limited in one direction by the actuating surface 33 of theseparating valve 5 and of the pressure relief piston 7 are connectedwith one of the sections 1 1, 12 of the cylinder bore by means of apressure medium line. The sections are separated from each other by thevalve body of a two-way valve 13 which is electro-magnetically operatedand designed as a double-acting cone valve. The sections 11, 12 of thecylinder bore which have to be separated from one another are formed bya recess in the surface of the valve body. In the center the latterprovides a valve closing member 14 which is conical on each side. Theseat for both valve cones is formed by the edges of an annular groove 15on the inner circumference of the cylinder bore. The annular groove 15communicates either with the section 11 or with the section 12 of thecylinder bore depending on the position of the valve body. From theannular groove 15 a pressure medium line 16 leads directly to thehydraulic energy source, e.g., an accumulator. A check valve 17 whichopens in direction of the cylinder chamber 6 is arranged in the pressuremedium line from the cylinder chamber 6 of the pressure relief piston 7to the section 12 of the cylinder bore of the two-way valve 13. Apressure medium line 18 connects the section 11 of the cylinder chamberof the valve 13 which communicates with the cylinder chamber containingthe actuating surface 33 of the separating valve 5, with the section 19of the cylinder chamber of a second solenoid two-way valve 20 designedin the same way as 13. A pressure medium line 22 from the cylinderchamber 6 of the pressure relief piston 7 leads into the section 23 ofthe cylinder chamber which is separated from the section 19 by the valveclosing member 21 of the valve 20 which is conical on both sides. Fromthe annular groove 24 whose edges form the valve seat for both theconical surfaces of the valve closing member 21 a pressure medium linebranches off and leads directly to the reservoir. Depending on theposition of the valve body the space of the annular groove 24communicates with one of the sections 19 or 23 of the cylinder chamberof the twoway valve 20.

In order to keep the brake system operative when the pressure supplyfrom the accumulator is lost an automatic locking device is providedwhich holds the solenoid two-way valve 20 in initial position when theaccumulator pressure fails. The control is then bypassed and normalbraking is possible.

From the pressure medium channel 16 which is directly connected with theaccumulator a connection 25 leads into a chamber 26 which is formed by acylindrical recess 28 in the housing 3 and separated into two parts by apartition 27 arranged on a rod 29. On one side of the partition 27 therod 29 rests on the bottom of the second closed chamber 30 of thecylindrical recess 28 and with the other end it passes through thechamber 26, connected with the accumulator, and from there it projectsthrough a connecting bore 31 into the part of the cylinder chamber ofthe valve 20 which controls the connection to the reservoir. In saidpart a spring is arranged which overcomes the frictional force of thesealings of the valve body. Between the opposite surfaces of the rod 29and the valve body a gap is formed in normal position and insures areliable opening and closing of the two-way valve 20. The chamber 26which is connected with the accumulator is sealed against the passage 31at the bottom and at the partition 27 by sealing sleeves to prevent theaccumulator pressure from penetrating into the second chamber 30 or thecylinder chamber of the two-way valve 20.

In initial position the pressure of the accumulator in chamber 26presses the partition 27 with the rod 29 against a spring 32 arranged inthe chamber 30. Said spring is propped between the partition 27 and thecylinder bottom.

OPERATION In the rest position the master cylinder is directly connectedwith the wheel cylinder by the open separating valve 5. The pressurerelief piston 7 rests with the flange 9 on the shoulder 10 of itscylinder chamber 6 because its actuating surface 8 is under pressurefrom the accumulator. This pressure comes into the cylinder chamber 6through the line 16 and the solenoid valve 13 via a check valve 17. Inthe rest position the solenoid valve 13 connects the annular groove 15where the line 16 from the accumulator ends with the section 12 of thecylinder chamber of the valve 13 from which a connection is provided tothe cylinder chamber 6. The chamber of the cylinder bore 4 of theseparating valve 5 containing the actuating surfaces 33 of the pressuremedium operated valve body communicates with the reservoir throughpressure medium lines and through the sections 11, 19 of the solenoidvalves 13, 20, the sections being separated from the accumulatorpressure. The section 23 of the cylinder chamber of the valve 20 isconnected via the pressure medium line 22 with the cylinder chamber 6 ofthe pressure relief piston 7, the chamber being under pressure from theaccumlator, and is separated in rest position of the valve 20 by a valveclosing member 21 from the annular groove 24 which communicates with thereservoir.

Therefore in the rest position of the system, brake pressure can bebuilt up in the wheel cylinder from the master cylinder. When the dangerof skidding arises the wheel sensor generates a signal which is strongenough to start the control cycle. The windings of the two solenoidvalves 13, 20 are supplied with current and both move into theiractuated positions. The annular groove 15 of the valve 13 controllingthe accumulator pressure is connected with the section 11 of the valvespace from which a pressure medium line leads to the actuating surface33 of the separating valve 5. The cylinder chamber 6 of the pressurerelief piston 7 is separated from the accumlator by the valve cone 14.At the same time the cylinder chamber 6 is connected with the return bya switch-over of the solenoid valve 20 controlling the reservoir via thepressure medium channel 22, the section 23 of the valve chambercommunicating with the annular groove 24 while the cylinder chamber ofthe separating valve 5 is separated from the reservoir by the valveclosing member 21.

The separating valve 5 is closed under the accumulator pressure appliedto the actuating surface 33 of the valve body so that wheel cylinder andmaster cylinder are separated from one another. The pressure medium inthe cylinder chamber 6 of the pressure relief piston 7 expands into thereservoir and the pressure relief piston 7 moves into the cylinderchamber 6, under the influence of the brake pressure in section 2 of thebrake line connected with the wheel cylinder. In this way the volume ofthe pressure medium line 2 increases so that the brake pressure isreduced and the respective wheel is reaccelerated to overcome thecritical deceleration range. When the signal of the sensor disappearsthe exciting current of the solenoid valves 13, is interrupted and bothare returned to their initial position by the force of the returnspring. in this way the cylinder chamber 6 of the pressure relief piston7 is again connected to the accumulator and separated from thereservoir, so that the pressure relief piston 7 returns to its initialposition and the original brake pressure is built up again. The pressureexerted on the front face (actuating surface) 33 of the valve body ofthe separating valve 5 expands into the reservoir so that the separatingvalve 5 combined with a check valve is opened by the force of the returnspring and the connection between the master cylinder and the wheelcylinder is restored. A new braking process and a new control cycle canbe started.

In order to insure a reliable operation of the brake system even whenthe pressure from the accumulator fails the above mentioned lockingdevice is installed. When the accumulator pressure fails the pressure inthe cylinder chamber 6 has to be maintained in order to avoid anincrease of volume of the brake circuit by the movement of the pressurerelief piston 7. This is partly effected by the check valve 17 which isarranged in the line from the cylinder chamber 6 to the valve 13controlling the accumulator pressure and partly by keeping the valve 20which controls the return in the initial position by the locking deviceso that the pressure medium cannot flow from the cylinder chamber 6through the pressure medium line 22 to the reservoir. When theaccumulator pressure fails the pressure in the chamber 26, connectedwith the accumulator, drops and the prestressed spring 32 moves thepartition 27 with the rod 29 so that the volume of the chamber 26 isreduced and the part of the rod 29 projecting into the cylinder chamberof the solenoid valve comes to lie against the front face of the valvebody and thus keeps the valve 20 in its initial position even when thesensor of the control system gives off a signal which would initiate acontrol cycle. The force of the spring 32 has to be larger than theelectromagnetic force which may originate from the exciting winding ofthe valve 20. When the accumulator pressure returns the locking deviceautomatically returns to its rest position.

A special advantage of this invention is that a strongly prestressedspring for the constructional unit effecting the volume increase in thebrake circuit can be avoided.

Due to the mechanical separation of the separating valve and thepressure relief piston the volume increase is always comparable with thecontrol signal so that no underpressure or vacuum will result in thebrake line. v

This prevents foaming and the consequent loss of brake pressure.Compared to known antiskid control devices the device according to theinvention is essentially smaller.

While I have described above the principles of my invention inconnection with specific apparatus it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the accompanying claims.

I claim:

1. An antiskid brake control system in which the hydraulic connectionbetween the wheel cylinder and the master cylinder is interrupted by aseparating valve in response to a brake release signal generated by anantiskid sensor system and in which the hydraulic brake fluid can expandinto a chamber connected to the wheel cylinder by a volume increase upona controlled movement of a pressure relief piston, the system beingcharacterized in that upon a response of the separating valveinterrupting the brake line the movement of the pressure relief pistoncausing the volume increase is effected independently from theseparating valve and both directions of movement by a controlleddifference of hydraulic pressure applied to the actuating surfaces ofthe pressure relief piston;

the part of the respective cylinder chambers containing the actuatingsurface of the separating valve and the actuating surface of thepressure relief piston can alternatively be connected to a hydraulicpressure source and to a reservoir by a pair of two-way valves, thecylinder bore for the separating piston communicating with the portionof the brake line connected with the wheel cylinders and the separatingpiston being guided in its cylinder bore in a sealing manner;

said two-way valves being electromagnetically operated double-actingcone valves each having a valve chamber divided into three sections byrecesses in the valve body and by an annular groove in the center of thecylinder bore of the respective two-way valve; the sections of therespective valve chamber being formed by the recesses are separated by avalve closing member centrally guided on the valve body and have aconical shape on each side; the edges of the annular groove form theseat for the valve closing member and the annular groove communicateswith either one or the other section depending on the position of thevalve body.

2. An antiskid brake control system as defined in claim 1, characterizedin that the sections of the valve chambers of the twoway valvescommunicate with each other and with the cylinder chamber containing theactuating surface of the separating valve by means of hydraulic conduitsand that the sections of the valve chambers of the two valves are eachseparately connected with the cylinder chamber of the pressure reliefpiston by means of hydraulic conduits and that the annular groove of thevalve communicates with a hydraulic pressure source, and the annulargroove of the valve with the reservoir.

3. An antiskid brake control system as defined in claim 2, characterizedin that in rest position of the system the section of the valveconnected with the cylinder chamber of the pressure relief pistoncommunicates with its annular groove and the section of the valveconnected with the cylinder chamber of the separating valve communicateswith its annular groove so that the cylinder chamber of the pressurerelief piston is in communication with the hydraulic pressure source andthe cylinder chamber of the separating valve communicates with thereservoir.

4. An antiskid brake control system as defined in claim 3, characterizedin that a check valve opening towards the cylinder chamber is arrangedin the hydraulic conduit from section of the valve to the cylinderchamber of the separating piston.

5. An antiskid brake control system as defined in claim 4, characterizedin that a cylindrical recess is divided into two parts by a partitionworked onto a pin, the pin projecting through a connecting bore into thecylinder chamber of the valve controlling the return and keeping thepartition in initial position against a prestressed spring by a pressurefrom the accumulator conveyed into the chamber facing the valve so thata gap between the opposite surfaces of the pin and the valve body of thevalve remains open and ensures a reliable opening and closing of thevalve.

6. An antiskid brake control system comprising:

skid sensing means for generating an electrical skid signal in responseto an incipient skid condition of a vehicle wheel;

a master cylinder;

a brake for at least one wheel;

a first pressure medium line connected between said master cylinder andsaid brake;

a separating valve connected in said first pressure medium line; saidseparating valve having a first pressure medium actuating surface, anormally open position to connect said master cylinder to said brake andan actuated closed position to disconnect said master cylinder from saidbrake;

a first hollow cylinder connected in said first pressure medium lineindependent of said separating valve;

a pressure relief piston slidably sealed in said first hollow cylinder,said piston having a second pressure medium actuating surface, a firstposition to provide a desired volume in said first pressure medium lineand a second position to increase the volume of said first pressuremedium line;

a pressure medium accumulator;

a pressure medium reservoir;

a second pressure medium line connected to said accumulator;

a first two-way solenoid valve connected to said second pressure mediumline, said first valve having an unactuated position to apply pressuremedium from said accumulator to said second actuating surface to placesaid piston in said first position and an actuated position to connectthe pressure medium from said accumulator to said first actuatingsurface to place said separating valve in said actuated closed position;and

a second two-way solenoid valve having a first input port connected tosaid first hollow cylinder adjacent said second actuating surface, asecond input port connected to said separating valve adjacent said firstactuating surface and a single output port connected to said reservoir,said second solenoid valve having an unactuated position to connect saidsecond input port to said output port and an actuated position toconnect said first input port to said output port;

said first and second solenoid valve being coupled in common to saidskid sensing means and actuated simultaneously by said electrical skidsignal.

lh aili nifififi iii fliifii li liflhi to permit normal operation ofsaid second solenoid valve when said accumulator is operating normally,but to prevent said second solenoid valve from achieving said actuatedposition in the presence of said electrical skid signal ifsaidaccumulator fails thereby enabling normal braking operation.

8. A system according to claim 6, wherein each of said first and secondsolenoid valves include further including a rod disposed coaxially ofsaid second hollow cylinder of said second solenoid valve, one end ofsaid rod being in spaced relation with an adjacent end of said piston ofsaid second solenoid valve,

a spring urging said rod toward said piston of said second solenoidvalve, and

an arrangement associated with said rod within said second hollowcylinder of said second solenoid valve, said arrangement being connectedto said accumulator to overcome the urging of said spring when saidaccumulator is operative, but which will enable said rod to engage theadjacent end of said piston of said second solenoid valve to preventsaid second solenoid valve from achieving said actuated position in thepresence of said electrical skid signal if said accumulator failsthereby enabling normal braking operation.

1. An antiskid brake control system in which the hydraulic connectionbetween the wheel cylinder and the master cylinder is interrupted by aseparating valve in response to a brake release signal generated by anantiskid sensor system and in which the hydraulic brake fluid can expandinto a chamber connected to the wheel cylinder by a volume increase upona controlled movement of a pressure relief piston, the system beingcharacterized in that upon a response of the separating valveinterrupting the brake line the movement of the pressure relief pistoncausing the volume increase is effected independently from theseparating valve and both directions of movement by a controlleddifference of hydraulic pressure applied to the actuating surfaces ofthe pressure relief piston; the part of the respective cylinder chamberscontaining the actuating surface of the separating valve and theactuating surface of the pressure relief piston can alternatively beconnected to a hydraulic pressure source and to a reservoir by a pair oftwo-way valves, the cylinder bore for the separating pistoncommunicating with the portion of the brake line connected with thewheel cylinders and the separating piston being guided in its cylinderbore in a sealing manner; said two-way valves being electromagneticallyoperated doubleacting cone valves each having a valve chamber dividedinto three sections by recesses in the valve body and by an annulargroove in the center of the cylinder bore of the respective two-wayvalve; the sections of the respective valve chamber being formed by therecesses are separated by a valve closing member centrally guided on thevalve body and have a conical shape on each side; the edges of theannular groove form the seat for the valve closing member and theannUlar groove communicates with either one or the other sectiondepending on the position of the valve body.
 2. An antiskid brakecontrol system as defined in claim 1, characterized in that the sectionsof the valve chambers of the two-way valves communicate with each otherand with the cylinder chamber containing the actuating surface of theseparating valve by means of hydraulic conduits and that the sections ofthe valve chambers of the two valves are each separately connected withthe cylinder chamber of the pressure relief piston by means of hydraulicconduits and that the annular groove of the valve communicates with ahydraulic pressure source, and the annular groove of the valve with thereservoir.
 3. An antiskid brake control system as defined in claim 2,characterized in that in rest position of the system the section of thevalve connected with the cylinder chamber of the pressure relief pistoncommunicates with its annular groove and the section of the valveconnected with the cylinder chamber of the separating valve communicateswith its annular groove so that the cylinder chamber of the pressurerelief piston is in communication with the hydraulic pressure source andthe cylinder chamber of the separating valve communicates with thereservoir.
 4. An antiskid brake control system as defined in claim 3,characterized in that a check valve opening towards the cylinder chamberis arranged in the hydraulic conduit from section of the valve to thecylinder chamber of the separating piston.
 5. An antiskid brake controlsystem as defined in claim 4, characterized in that a cylindrical recessis divided into two parts by a partition worked onto a pin, the pinprojecting through a connecting bore into the cylinder chamber of thevalve controlling the return and keeping the partition in initialposition against a prestressed spring by a pressure from the accumulatorconveyed into the chamber facing the valve so that a gap between theopposite surfaces of the pin and the valve body of the valve remainsopen and ensures a reliable opening and closing of the valve.
 6. Anantiskid brake control system comprising: skid sensing means forgenerating an electrical skid signal in response to an incipient skidcondition of a vehicle wheel; a master cylinder; a brake for at leastone wheel; a first pressure medium line connected between said mastercylinder and said brake; a separating valve connected in said firstpressure medium line; said separating valve having a first pressuremedium actuating surface, a normally open position to connect saidmaster cylinder to said brake and an actuated closed position todisconnect said master cylinder from said brake; a first hollow cylinderconnected in said first pressure medium line independent of saidseparating valve; a pressure relief piston slidably sealed in said firsthollow cylinder, said piston having a second pressure medium actuatingsurface, a first position to provide a desired volume in said firstpressure medium line and a second position to increase the volume ofsaid first pressure medium line; a pressure medium accumulator; apressure medium reservoir; a second pressure medium line connected tosaid accumulator; a first two-way solenoid valve connected to saidsecond pressure medium line, said first valve having an unactuatedposition to apply pressure medium from said accumulator to said secondactuating surface to place said piston in said first position and anactuated position to connect the pressure medium from said accumulatorto said first actuating surface to place said separating valve in saidactuated closed position; and a second two-way solenoid valve having afirst input port connected to said first hollow cylinder adjacent saidsecond actuating surface, a second input port connected to saidseparating valve adjacent said first actuating surface and a singleoutput port connected to said reservoir, said second solenoid valvehaving an unactuated positIon to connect said second input port to saidoutput port and an actuated position to connect said first input port tosaid output port; said first and second solenoid valve being coupled incommon to said skid sensing means and actuated simultaneously by saidelectrical skid signal.
 7. A system according to claim 6, furtherincluding an arrangement connected to said accumulator to permit normaloperation of said second solenoid valve when said accumulator isoperating normally, but to prevent said second solenoid valve fromachieving said actuated position in the presence of said electrical skidsignal if said accumulator fails thereby enabling normal brakingoperation.
 8. A system according to claim 6, wherein each of said firstand second solenoid valves include a second hollow cylinder having anannular radially extending portion, and a piston slidably sealed to theinner surface of said second hollow cylinder including a double conicalvalve member disposed for movement within said annular radiallyextending portion, the inner edges of said annular radially extendingportion providing valve seats for said conical valve members; andfurther including a rod disposed coaxially of said second hollowcylinder of said second solenoid valve, one end of said rod being inspaced relation with an adjacent end of said piston of said secondsolenoid valve, a spring urging said rod toward said piston of saidsecond solenoid valve, and an arrangement associated with said rodwithin said second hollow cylinder of said second solenoid valve, saidarrangement being connected to said accumulator to overcome the urgingof said spring when said accumulator is operative, but which will enablesaid rod to engage the adjacent end of said piston of said secondsolenoid valve to prevent said second solenoid valve from achieving saidactuated position in the presence of said electrical skid signal if saidaccumulator fails thereby enabling normal braking operation.